1. Field of the Invention
The present invention relates to a recording apparatus and method, or more particularly, to an ink-jet recording apparatus and method for recording by discharging ink from a recording head.
2. Description of the Related Art
Recording apparatuses such as printers, copying machines, and facsimile machines are designed to record images formed with dot patterns on recording materials such as paper or plastic film according to image information.
When the recording apparatuses are classified in terms of recording modes, they are of an ink-jet type, a wire-dot type, a thermal type, and a laser beam type. The ink-jet type (ink-jet recording apparatus) is designed to effect recording by discharging ink (recording agent) droplets through ink-jet outlets in a recording head and fusing the ink droplets onto a recording member.
In recent years, numerous recording apparatuses have been put to use. These recording apparatuses are being demanded to offer high recording speeds, high resolution, high image quality, and low noise. The ink-jet recording apparatus is a recording apparatus capable of meeting these demands. The ink-jet recording apparatus effects recording by discharging ink from a recording head. Stabilization of ink discharging and quantity (volume) of discharged ink is therefore essential to satisfy the above demands.
The ink-jet recording apparatus has a facility for stabilizing ink discharging. Nevertheless, definition in recorded images depends largely on the performance of an individual recording head. A very small difference of a recording head from others occurring in the process of manufacturing recording heads; such as, a difference in the shape of each ink-discharging outlet (port, orifice) of a recording head or a difference in performance of an electro-thermal converter (discharge heater) affects a quantity of discharged ink or a discharge direction. Consequently, nonuniformity of density occurs in a finalized image, thus deteriorating image definition.
An example of the foregoing problem will be described in conjunction with FIGS. 11-a-11-c and 12-a-12-c. In FIG. 11-a, 1101 denotes a multiple-nozzle recording head or multi-head. For simple illustration, the multi-head shall include eight nozzles 1102. Reference numeral 1103 denotes ink droplets discharged by the multiple nozzles 1102. Normally, it is ideal that ink is discharged with equal quantities of discharged ink in the same direction. If this ink discharging is carried out, as shown in FIG. 11-b, dots of the same size are printed on paper to produce a uniform image unaffected with nonuniformity of density (11-c).
In practice, however, as described previously, nozzles differ from one another. If printing is executed as described above with the nozzles left unchanged, as shown in FIG. 12-a, sizes and orientations of ink droplets discharged by the nozzles differ from one another. The ink droplets are shot at paper as shown in FIG. 12-b. As illustrated, a blank space appears at regular intervals in a main scanning direction of a recording head because ink does not occupy a pixel 100%. Otherwise, dots overlap enormously, or a white band (stripe) runs as shown in the center of FIG. 12-b. Thus-printed dots result in the density distribution shown in FIG. 12-c along the nozzle array. As a result, these phenomena are perceived as nonuniformity of density by normal human eyes.
A method described in, for example, Japanese Patent Laid-Open No. 60-107975 has been devised as a solution of the nonuniformity of density. The method will be described in conjunction with FIGS. 13-a-13-c and 14-a-14-c. According to the method, a multi-head 2001 is scanned three times in order to furnish a print area shown in FIGS. 11-b and 12-b. A unit-area of four pixels is furnished by making two passes. Eight nozzles of the multi-head are grouped into four upper nozzles and four lower nozzles. A dot printed by one nozzle during one scan is based on defined image data that has been thinned out according to a certain image data array so as to be about half in amount. Another dot is then printed based on the remaining half of the image data during the second scan. Thus, printing is completed for the unit area of four pixels. The foregoing recording method is referred to as a multi-pass recording method.
When the multi-pass recording method is adopted, even if a multi-head similar to that shown in FIG. 12-a is employed, since influences inherent to the nozzles upon a print image are halved, a printed image appears as shown in FIG. 13-b. A black stripe and a white stripe shown in FIG. 12-b become inconspicuous. The nonuniformity of density shown in FIG. 12-c is considerably alleviated as shown in FIG. 13-c.
When the above recording is carried out, image data is grouped into two complementary portions to be assigned to the first and second scans according to a predetermined array. The image data array (thinning-out pattern) is usually, as shown in FIG. 14-a, a checker pattern in which image data is allocated to every other pixel location lengthwise and sideways.
Printing of a unit print area (of four pixels deep) is therefore completed by performing the first scan during which a checker pattern is printed and the second scan during which an inverse-checker pattern is printed.
Electric control for the foregoing thinned-out printing is illustrated in FIGS. 15 and 16. When print data Si is placed in an 8-bit shift register in response to a print data synchronizing clock CLK and then signals BEI1*, BEI2*, BEI3*, and BEI4* are turned on, in a head unit 28 a transistor array 26 is driven so that a heater 27 generates heat. Thus, printing is effected. Herein, the asterisk * indicates that the signal is low active. A signal LATCH* is a control signal for latching print data. A signal CARESI* is a reset signal for clearing a latch 25. Every heating is initiated with a Heat Trigger signal. A pulse generator 23 outputs the signals BEI1*, BEI2*, BEI3*, and BEI4*. These signals may sometimes be output with time lags between adjoining ones. Herein, they are supposed to be output simultaneously.
For thinning out, a flip-flop 22 shown in FIG. 15 is triggered with the Heat Trigger signal so that signals (for example, the signals BEI1* and BEI3*) to be masked are alternately changed at every heating. In reality, the signals to be masked are changed when an output signal DATA ENB of the flip-flop 22 is driven high or low according to a timing chart shown in FIG. 16. With the Heat Trigger signal, the signals BEI1*, BEI2*, BEI3*, and BEI4* are driven low. Respective nozzles are heated. Dotted lines in FIG. 16 indicate masking durations which occur in line with the cycle time of the signal DATAENB. Signals EVEN and ODD are used for initialization of a mask. When printing is to be performed using a checker-pattern mask, the signal EVEN is fed prior to printing of one line. The flip-flop 22 is then pre-set, whereby printing based on a checker-pattern mask is enabled. For a line on which printing is to be performed using an inverse-checker pattern mask, the signal ODD is fed to pre-set the flip-flop 22. The signals BEI2* and BEI4* are turned on earlier, whereby printing based on an inverse-checker pattern mask is enabled.
FIGS. 14-a, 14-b, and 14-c show how recording of a certain area is completed by applying checker-pattern and inverse-checker pattern masks using a multi-head having eight nozzles as that shown in FIG. 13-a. First, during the first scan, four lower nozzles are used to create a checker pattern (hatched circles) (FIG. 14-a). Next, during the second scan, paper is fed by a quantity correspondent with a depth of four pixels (half of a head length). An inverse-checker pattern (white circles) is then created (FIG. 14-b). During the third scan, the paper is fed by a quantity correspondent with a depth of four pixels (half of the head length) again. A checker pattern is then created (FIG. 14-c).
As mentioned above, paper feed is performed in units of four pixels, and checker-pattern and inverse-checker pattern masks are used alternately. A record area of four pixels deep is thus produced with each scan. As described above, two types of nozzles are used for the same area in order to complete printing of the area. This results in a high-quality image unaffected with density nonuniformity. However, even when the foregoing multi-pass recording is adopted, the density nonuniformity may not be eliminated depending on a duty ratio. Especially in half-tone recording, new density nonuniformity may be identified. The phenomenon will be described below.
In general, image data to be recorded in a certain area and received by a printer is regularly formatted as an array. A recording apparatus stocks or stores a certain amount of data in buffers, and applies a new mask having the aforesaid checker or inverse checker pattern (image array pattern) to the data. When the associated pixel locations in the data and mask are turned on, the associated pixels are printed.
FIGS. 17 to 19 explain the above recording procedure. In FIG. 17, 1710 denotes data having been arrayed and placed in a buffer. Reference numeral 1720 denotes a checker-pattern mask indicating locations of pixels allowed to be printed during the first pass. Numeral 1730 denotes an inverse-checker pattern mask indicating locations of pixels allowed to be printed during the second pass. Numerals 1740 and 1750 are illustrations showing pixels to be printed during the first and second passes respectively.
In FIG. 17, arrayed data is stocked in a buffer for 25% of a certain area. The data is usually such print data that is scattered to the greatest extent in an effort to keep the density in the certain area uniform. The fashion of arraying image data depends on what kind of area gray scale is employed for image data processing to be performed before the image data is transferred to a printer. Numeral 1710 denotes a example of an data array of 25% image data. When the data is printed by applying the masks 1720 and 1730, pixels representing exact halves of the original data are recorded as shown in the illustrations 1740 and 1750 after the first and second passes respectively.
However, as shown in FIG. 18, when 50% image data comes in, it is quite probable that data 1810 dispersed to the greatest extent may be consistent with a checker pattern mask 1820 or an inverse-checker pattern mask 1830.
When such an event occurs, printing of all the image data is completed after the first pass (1840). No recording is therefore performed during the second pass (1850). That is to say, the same nozzles are responsible for all the print data 1810. An adverse influence derived from the differences of the nozzles from one another is reflected as density nonuniformity . The fundamental object of the aforesaid division recording is not accomplished.
FIG. 19 shows printed states of arrayed image data offering a higher duty ratio than those shown in FIGS. 17 and 18. As apparent from FIG. 19, the number of printed pixels differs considerably between the first and second passes. Nonuniformity of density which is suppressed at a high duty ratio close to 100% recurs at a low duty ratio below 50%.
Consideration will be taken with regard to printing on transparent film under these circumstances. Printing is completed by making the first and second passes so that as many adjoining dots as possible will not be printed simultaneously. This is intended to prevent occurrence of beading. Nevertheless, the aforesaid print state ensues. This means that the advantage of division printing is not exerted because of a combination of a specific dither pattern and a print pattern. Beading is conspicuous in an area of a produced image corresponding to the combined portion of the patterns. If gradation is printed, a quite unpleasant texture appears in the area of the image.
In FIG. 14, the head always uses all the nozzles to print either the checker or inverse checker pattern. As for an upper half of the print area shown in FIG. 14 having a depth of four pixels, a checker pattern is first printed and an inverse checker pattern is then printed. As for a lower half thereof having a depth of four pixels, an inverse checker pattern is first printed and a checker pattern is then printed. When this printing procedure is discussed in conjunction with the aforesaid problem, it is deduced that a print area in which many dots are created during the first pass and a few dots are created during the second pass, and a print area in which almost no dots are created during the first pass and quite a few dots are created during the second pass appear alternately every other half of the length of a recording head. This phenomenon poses a problem, which will be described below, to occur along a border between print areas during ink-jet recording.
In the ink-jet recording method, when a dot is superposed on a previously recorded dot, the dot deposited after the previously-recorded dot in the superposed area tends to expand in the depth direction of paper.
FIG. 20 is a sectional view schematically showing the expansion. A pigment such as a dye contained in discharged ink is physically and chemically coupled with a recording medium. At this time, the coupling of the pigment with a recording medium P is definite. As long as the coupling force does not differ among pigment types, the coupling of a pigment I1 (crosshatched in FIG. 20) of previously-discharged ink with a recording medium is given priority and therefore mostly left on the surface of the recording medium P. A pigment I2 (hatched in FIG. 20) of ink discharged later is hardly coupled with the recording medium P on the surface of the recording medium P and therefore expands into the recording medium. When a reaction of ink is considered on the level of fibers of the recording medium P, once fibers are coupled with the dye in ink, the fibers are more hydrophilic than they are when they are not coupled therewith. Ink droplets shot (landed) at an area adjacent to a highly hydrophilic area of the recording medium are liable to be attracted toward the area at which previously-discharged ink droplets are shot.
When preceding ink droplets are fused insufficiently, that is, when preceding ink droplets contain more water, an area of a recording medium at which the ink droplets are shot is more hydrophilic. More ink droplets shot at an area adjacent to the area are therefore liable to be attracted to the previous area. When a print area in which many dots are created first and then a few dots are created, and a print area in which almost no dots are created first and then quite a few dots are created during the second pass appear alternately every other half of the length of a recording head, dots printed on the margin of the print area adjacent to the print area at which much ink has been shot are easily attracted, while dots printed on the margin of the print area adjacent to the print area in which little ink has been shot are hardly attracted. Due to this difference in attraction, a high density area and a low density area are created on the border between the print areas. This results in density nonuniformity. The density nonuniformity becomes conspicuous especially in half-tone recording and has such periodicity that density nonuniformity appears every other half of the length of a recording head.
When a specific mask is used to effect thinning-out printing, print data may have the same periodicity as the mask. In other words, the amplitude of density defined with the arrangement of print pixel locations and non-print pixel locations in a mask may be consistent with the amplitude of print data and then be resonant therewith. As a result, a dot array is formed to include a pattern oriented in a certain direction. In general, this phenomenon is referred to a moire pattern. When images on a plurality of lines are based on the same mask, the moire pattern is more conspicuous and more discernible by users. The moire pattern depends largely on the periodicity of a mask.
Due to the aforesaid problems, the multi-pass printing which has been adopted to correct differences of nozzles from one another does not always provide satisfactory image quality because of density nonuniformity. The density nonuniformity has such periodicity that it appears in every other print area having a certain depth. The periodicity facilitates human perception of discerning the density nonuniformity.
Next, consideration will be given to printing on a type of recording medium that is prone to beading and less absorptive, such as transparent film, but not printing on a type of recording medium that is quite absorptive, such as coated paper or plain paper.
If an area of a recording medium, which is prone to beading even in a normal condition, is twisted, the ink beads are stopped by the white stripe and enlarged to produce large streaky patches. This phenomenon is more critical in transparent film than in plain paper or coated paper. In an effort to solve this problem, a proposal has been made for a method of repeating recording a plurality of times in order to complete an image for one line.
However, according to the prior art, a thinning-out mask is a fixed mask. When a thinning-out mask which may cause a great difference between the number of print dots for the first pass and that for the second pass is employed, beading often varies its intensity especially during half-tone recording. That is to say, when a large number of dots are printed during a single pass, beading becomes more intense. The intensity of beading changes with the synchronism of a gray scale pattern with a thinning-out mask pattern for each pass. Aside from this case, when beading occurs between adjoining dots, a half-tone dither pattern may be enlarged or exaggerated to become a conspicuous unpleasant pattern, or a gray scale may be destroyed.
When the conventional fixed mask for regularly thinning-out is used, beading occurs askew and appears with exaggerated askewness because of an inaccurate mounting position of a recording head, unequal speeds of a carriage during a plurality of printing passes, different set positions of a carriage, different paper feed positions, and differences in ink discharging speed of a recording head. When the regularity of a fixed mask is made higher in order to eliminate the influence of beading or the mask size thereof is made larger, if the fixed mask is displaced from an ideal position due to an error, beading becomes more conspicuous. This depends on the pattern of a fixed mask, though. The aforesaid moire pattern is therefore enlarged.